A diffusion transfer process utilizing silver salts such as silver halide, etc., has been known. In such a photographic process, it is known that positive silver images are obtained directly on an image receiving element by putting a light-sensitive element containing exposed silver halide photographic emulsion on an image receiving element containing silver precipitants and processing in the presence of a developing agent by applying an alkaline processing solution containing a silver halide solvent between these two elements.
According to this process, unexposed silver halide emulsion in the light-sensitive element is dissolved by the silver halide solvent to come into the alkaline processing solution as a silver ion complex, which is transferred to the image receiving element, wherein it precipitates as silver images by the action of silver precipitants, and, consequently, direct positive images are formed.
The image receiving element used in this process is generally produced by providing an image receiving layer which contains silver precipitants selected from metal sulfides such as nickel sulfide, silver sulfide or palladium sulfide, etc., and noble metal colloids such as gold, silver, palladium, etc., in an alkali permeable polymer binder selected from gelatin, carboxymethyl cellulose, hydroxyethyl cellulose, regenerated cellulose, polyvinyl alcohol, sodium alginate, starch, gum arabic and colloidal silica, etc., on a paper support such as baryta paper, polyethylene laminated paper, lacquer paper or synthetic paper, etc., or a film support such as acetyl cellulose, polyethylene terephthalate or polystyrene film, etc.
In order to improve the image receiving elements, numberous inventions have been made. Particularly, it has been known to use regenerated cellulose as a binder in the image receiving layer.
Specifically, in U.S. Pat. No. 3,179,517, a process has been described for producing image receiving elements which comprises forming regenerated cellulose by hydrolyzing an acetyl cellulose film with alkali, and thereafter immersing the regenerated cellulose layer in a solution of a gold salt and a solution of a reducing agent to cause a reaction in the layer, whereby silver precipitant of gold colloid are formed. Further, in Japanese Patent Publication No. 32754/69, it has been described to produce an image receiving element by a process which comprises incorporating silver precipitant in an alkali impermeable polymer substance by vacuum deposition, dissolving it in a solvent which dissolves the polymer substance, applying the resulting solution to a support, drying and hydrolyzing the surface layer of the formed polymer layer so as to have an alkali permeable property.
Further, in Japanese Patent Publication No. 43944/71, it has been described to produce an image receiving element by a process which comprises forming silver precipitants in a solution of acetyl cellulose, applying it to a support and hydrolyzing acetyl cellulose to convert it into regenerated cellulose.
Furthermore, in Japanese Patent Publication No. 49411/76, it has been described to prepare an image receiving element by a process which comprises hydrolyzing a cellulose ester layer and incorporating silver precipitants in the hydrolyzed layer simultaneously with or after carried out hydrolysis.
Moreover, in U.S. Pat. No. 4,163,816, production of an image receiving element has been described by a process which comprises hydrolyzing acetyl cellulose in a solution with an acid to convert it into acetyl cellulose having a low degree of acetylation, and applying the solution to a support.
However, silver images formed on the image receiving elements obtained as described above have a fault in that they easily discolor or fade during preservation.
As a process for improving the above-described fault, it has been described in Japanese Patent Publication No. 5392/71, U.S. Pat. No. 3,533,789 and British Pat. No. 1,164,642 to apply a water soluble polymer solution containing an alkali neutralizing component to the surface of the resulting silver images. However, according to this process, the surface to which the aqueous solution of the polymer is applied requires a long time for complete drying. Therefore, it is impossible to arrange the prints in layers while drying, because the surface is sticky, and thus fingerprints and dust often adhere to the surface. Further, application of such a solution to the silver images is troublesome.
Japanese Patent Publication No. 44418/81 has disclosed image receiving elements comprising a support, (I) a hydrolyzable cellulose ester, polyvinyl ester or polyvinyl acetal layer containing a diffusible compound capable of changing properties of silver images, which becomes to have an alkali permeable property by hydrolysis, provided on the support, and (II) a regenerated cellulose layer containing silver precipitants provided on said layer. As the diffusible compounds capable of changing properties of silver images, organic mercapto compounds have been described. In this case, the diffusion transfer processing solution and the mercapto compound in layer I gradually diffuse into layer II and protect silver images formed in layer II, whereby discoloration and fading can be prevented.
In order to completely exhibit the effect of preventing discoloration and fading, it is necessary that the mercapto compound has sufficient ability of preventing discoloration and fading and it remains in layer I during preservation of the undeveloped image receiving element or during diffusion transfer processing, but it permeates into layer II from layer I after formation of silver images by the diffusion transfer processing to protect the images formed on the layer II. If the mercapto compound diffuses into the layer II from the layer I before conclusion of the diffusion transfer processing, development is restricted and optical densities of the transfer silver images on image receiving material deteriorate on the whole. Further, if diffusion of the mercapto compound is retarded too much, discoloration or fading of images occurs before the silver images are protected by the mercapto compound.
However, mercapto compounds described in Japanese Patent Publication No. 44418/81 have faults, in that they have insufficient ability of preventing discoloration and fading and cause discoloration or fading of images, and further they restrain development by diffusing into the layer II from the layer I during preservation of the undeveloped image receiving element to deteriorate optical densities of the transfer silver images.
Further, Japanese Patent Application (OPI) No. 120634/74 has disclosed image receiving elements which are produced with using homopolymers, copolymers and graft polymers of monoacrylate or monomethacrylate of polyhydric alcohols as a polymer layer containing a compound which changes properties of silver images.
However, compounds described in Japanese Patent Application (OPI) No. 120634/74 have faults similar to those of compounds described in Japanese Patent Publication No. 4418/81, that they have insufficient ability of preventing discoloration or fading of images and they cause decrease of optical densities of transfer silver images.
Further, British Pat. No. 1,276,961 has disclosed that 2-mercapto-1,3,4-triazole derivatives are used for obtaining stable silver images by the diffusion transfer process. Moreover, U.S. Pat. No. 3,655,380 has disclosed that 5-seleno-1,2,3,4-tetrazole derivatives improve the tone of silver images obtained by the diffusion transfer process so as to be neutral gray and they give stable silver images.
However, these compounds have a fault in that the effect of stabilizing silver images obtained by the diffusion transfer process is insufficient to cause discoloration or fading of images. Therefore, it has been desired to provide compounds having a good effect of stabilizing silver images.